The present invention is directed to material processing and device fabrication. More particularly, the invention provides reusable substrates for electronic device fabrication and methods for preparing such substrates. Merely by way of example, the invention has been applied to light emitting diodes. But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) have been widely used in various electronics applications. To fabricate light emitting diodes, substrates are often important. In order to select a proper substrate, various factors, such as characteristics of fabrication equipments and/or desired LED devices, should be taken into account.
Conventionally, sapphire (e.g., Al2O3), silicon and silicon carbide have been selected to make substrates for LED fabrication. For example, a mono-crystalline sapphire substrate has been used to fabricate blue light/white light/ultra-violet light LEDs, laser diodes, high-frequency microwave devices and high-voltage power devices. In particular, gallium nitride (e.g., GaN) thin films (e.g., with a band gap of about 3.4 eV) are often epitaxially formed on the mono-crystalline sapphire substrate for device fabrication.
Sapphire materials often exhibit high melting temperatures, high hardness, and strong resistance to chemicals. Manufacturing of such mono-crystalline sapphire materials can consume significant electric power. Moreover, it may take tens of days for crystal growth of mono-crystalline sapphire with only a low yield. Thus, mono-crystalline sapphire ingots often are expensive to produce. In addition, the process to manufacture sapphire wafers from sapphire ingots is usually complex. As such, it can be costly to make mono-crystalline sapphire substrates.
Furthermore, on a mono-crystalline sapphire substrate, one or more epitaxial layers (e.g., GaN) can be formed through various processes (e.g., metal organic chemical vapor deposition (MOCVD)) in order to form an epitaxial structure. Afterwards, devices can be fabricated on the epitaxial structure including the sapphire substrate. Then, the sapphire substrate with the fabricated devices often is cut into small pieces (e.g., less than 1 mm by 1 mm) before the devices are separated from the sapphire substrate by a laser lift-off process (LLO). The conventional LLO technique often cannot successfully separate devices from a sapphire substrate if the substrate has a large size (e.g., larger than 5 mm by 5 mm). In addition, after the LLO process, the small pieces of the sapphire substrate usually cannot be reused. Thus, the conventional device-fabrication process often incurs high cost and may cause significant substrate waste.
Hence it is highly desirable to improve the techniques of material processing and device fabrication.